Sheet feeders are known that use a drive means such as a belt or roller to remove a single sheet laterally from the bottom of a stack of sheets that rests on a feed deck. The bottom sheet resists separation from the stack owing to friction with the next sheet in the stack above it and with the non-moving parts of the feed deck below it. In order for the sheet to slide away from the stack, the grip between the sheet and the drive means must exceed those frictional forces. Ideally, that should be true whether the bottom sheet is weighed on by a full stack or by only a few sheets.
Friction between the moving sheet and the stationary parts of the feed deck has a tendency to cause scuff marks on the surface of the paper. That is a particular problem with the increasing use of digital printing on paper with a smooth surface.
Some sheet feeders have used suction drums to improve the grip between the drive means and the paper. The surface of a suction drum is perforated by holes and an air pump is provided to suck air from the interior of the drum so that the paper clings to the drum's surface. The air pump may also be used to direct a jet of air towards the edge of the stack in order to assist with separating the sheets of the stack. However, such air pumps add to the bulk and complexity of the sheet feeder as well as its running costs and noise levels.
A sheet feeder should be able to deliver the sheets as fast as possible, up to the maximum operating speed of any downstream processing apparatus. It should reliably deliver only a single sheet at a time because a misfeed in which two sheets are delivered together will at best not be processed properly downstream and may at worst jam the apparatus, causing a costly delay in workflow.